Abstract
Global expansion of cyanobacterial harmful algal blooms (CyanoHABs), fueled by synergistic effects of anthropogenic nutrient loading and climate change, poses escalating threats to aquatic ecosystems and water security. Despite the critical role of altered precipitation regimes as climate change multipliers, quantitative mechanisms linking rainfall patterns to CyanoHABs dynamics remain poorly resolved. Here, we integrate a 60-year (1961-2020) trend analysis of rainfall extremes with four years of high-frequency monitoring of phytoplankton dynamics through synergies of manual sampling and autonomous buoy networks in Lake Qiandaohu, a deep subtropical reservoir serving as a vital drinking water source in East China. By synthesizing meteorological, hydrological, and limnological datasets through Mann-Kendall trend analysis, Random Forest, and Partial Least Squares Path Modeling, we elucidate environmental drivers and phytoplankton responses to shifting rainfall regimes. Results reveal significant increases (P<0.05) in annual rainfall and rainstorm frequency over six decades, peaking in early summer during local agricultural fertilization. Intensified early summer rainstorms triggered disproportionate phosphorus fluxes, lowering N/P ratios and transiently suppressing cyanobacterial dominance. However, post-rainfall stabilization of thermal stratification and residual nutrient enrichment selectively favored cyanobacterial proliferation within 20 days. Critically, extreme rainfall events delayed the onset of continuous dry-hot periods to late summer, creating prolonged thermal stratification ecological conditions for irreversible cyanobacterial dominance and blooms formation. Projected intensification of rainfall extremes necessitates integrated strategies combining watershed nutrient control, real-time meteorological forecasting, and targeted hydrodynamic interventions to mitigate CyanoHAB risks in climate-sensitive reservoirs.